1. Field of the Invention
The invention relates to wireless signal transceivers, and more particularly to frequency offset estimation (FOE) and automatic frequency control (AFC) for a filtered signal with destroyed phase information.
2. Description of the Related Art
Before a raw signal is transmitted, a signal transmitter modulates the raw signal with a carrier wave with a transmitting frequency suitable for air transmission to generate a radio signal. The signal transmitter transmits the radio signal through the air. A signal transceiver then receives the radio signal and demodulates the radio signal with a local wave with a receiving frequency to recover the raw signal. The receiving frequency of the local wave of the signal transceiver is assumed to be equal to the transmitting frequency of the carrier wave of the signal transmitter. However, in practice, there is unavoidably a tiny frequency difference between the receiving frequency of the signal transceiver and the transmitting frequency of the signal transmitter, and the frequency difference, referred to as frequency offset, degrades quality of the recovered raw signal. The signal transceiver therefore estimate a frequency offset for compensation before the recovered raw signal is further processed in the signal transceiver.
When a signal is filtered, the signal is often divided into an inphase component and a quadrature-phase component for further processing. If a filter filters the original signal according to different filter coefficients to obtain an I-component and a Q-component, the phase and frequency information is lost and cannot serve as a source for conventional frequency offset estimation. A conventional frequency offset estimation module therefore estimates a frequency offset value according to the original signal prior to filtration.
Referring to FIG. 1, a block diagram of a signal transceiver 100 comprises a channel estimator 102, an enhanced receiver 104, an equalizer 106, a channel decoder 108, and a conventional frequency estimator 110. The signal transceiver 100 receives an original signal X. The channel estimator 102 estimates a channel response of the original signal X. The enhanced receiver 104 is actually a filter filtering the original signal X to obtain a filtered signal Y with a carrier-to-interference (C/I) ratio higher than that of the original signal X. The equalizer 106 then equalizes the filtered signal Y to obtain an equalized signal Z, and the channel decoder 108 decodes the equalized signal Z to obtain raw data.
The conventional frequency offset estimator 110 estimates a frequency offset value Δfconv according to the original signal X prior to filtration as it cannot derive a frequency offset value from the filtered signal Y. The filtered signal Y, however, has a higher C/I ratio than that of the original signal X. The conventional frequency offset estimator 110 may fail to obtain the actual frequency offset value when the interference power becomes large, which might not be the case if utilizing filtered signal Y as the enhanced receiver 104 may effectively suppress certain interference. Since the frequency offset estimator 110 estimates the frequency offset value Δfconv based on the original signal X with a low C/I ratio, Δfconv is less accurate and cannot properly compensate the frequency drift, which degrades performance of the signal transceiver 100. In addition, when the C/I ratio of the original signal X is very low, the conventional frequency offset estimator 110 estimates a frequency offset value Δfconv dominated by the interference frequency offset with an inverse sign of the actual value, which causes divergence of automatic frequency control. The enhanced receiver 104 typically still could be operated at such a low C/I, the conventional frequency offset estimator 110 becomes a bottleneck of the overall transceiver 100.